The present invention is directed to improving the melt properties of polymers by forming polymeric blends using solid state shear pulverization. In particular, it has been found that polymeric blends can be formed by solid state shear pulverization, to yield moldable materials which have excellent molding processability without sacrificing and often improving melt strength. The formed polymeric blends have particular use in blow molding such as in forming blow molded films as well as in other mold processing, such as injection, extrusion, and rotational molding.
It is well-known to shape thermoplastic polymers by melt processing. Among the most common melt processing techniques are injection and extrusion molding of objects including those of relatively complex shape and blow molding 3-dimensional objects such as bottles as well as forming films. During injection molding, a polymer, usually in the form of pellets is melted in a screw extruder and the melted polymer is pushed through one or more gates into a mold cavity having a configuration of the article to be formed.
The mold gates are hollow passages in the mold which communicate with the mold cavity and are provided in numbers sufficient to allow the molten polymer to fill every crevice of the mold cavity. An injection mold cycle, thus, involves the injection of the molten polymer through the gates and into the mold cavity, hardening of the molten polymer sufficient to be free-standing once the mold is removed and final removal of the mold and cooling. Obviously, on a commercial scale, it is preferred to reduce the injection mold cycle time and increase production of molded objects. An important component of the cycle time is the flow of the molten polymer through the mold gates and into the mold cavity to completely fill same. Thus, it would be desirable to have a polymer which has a high melt flow rate to reduce the cycle time and further to insure that all crevices or complex portions of the mold cavity are properly filled less the molded article which is formed be incomplete. Molded articles of increasing complexity can be made if the polymer can readily fill the mold cavity. Unfortunately, simply using polymers which have high melt flow rates most often results in the sacrifice in tensile strength, impact strength, and other physical properties of the molded product as polymers which have a high melt flow rate typically have a low molecular weight and consequent lower physical properties. In extrusion molding, the polymer is melted and conveyed by a rotating screw through a die at the end of the extruder barrel which shapes the molten polymer into the desired object. Again, polymers with high melt flow rates are desired to reduce production time. However, polymers with high melt flow rates are not necessarily advantageous since these lower molecular weight materials may not be self-supporting after being shaped at the extrusion die face and often provide the molded object with less than desirable physical properties.
The compromise between melt flow rate and physical properties is no less critical, and most likely more critical in melt shaping a polymeric material by the blow molding process. In blow molding, a molten resin is either extruded or injected into the form of a tube which is expanded by air or other gas into a parison or bubble which is shaped by clamping a mold around the molten parison if an object such as a bottle is to be formed or the parison can be cut or passed between the nip of rollers to form thin films. In order to take advantage of blow molding techniques, a polymer must have sufficient melt strength to be blow moldable into an object. Not only must the polymer have the physical, tensile and thermal properties necessary for specific end use applications, the polymer must have a sufficient melt strength such that the polymer can support its own weight in the molten state after being extruded or injected into a parison. Thus, polymers have sufficient melt strength when the polymers can be extruded downward into the shape of the desired parison without the molten parison breaking. It is desirable to use polymers which are viscous, have a relatively low melt flow rate and which can be self-supporting in the form of the molten parison. On the other hand, polymers which have a low melt flow rate are not readily injected or extruded into the parison, and in the case of blown films, the thickness of the film may be excessive, degrading not only the overall properties of the film but degrading the economics of the blow molding process by increased material costs. Thus, as in injection and extrusion molding, it would be desirable to increase the melt flow rate of the molten polymer so as to improve productivity and form thinner films. At the same time, the melt strength of the polymer must be maintained so as to sustain the parison.
A relatively new type of molding process being increasingly used is rotational molding. In this molding process, a moldable polymer powder is placed within a hollow mold. The hollow mold is simultaneously heated and rotated to melt the powder and spread and evenly coat the inner mold surface with the molten polymer. In this molding process, it would be preferred that the molten polymer have a sufficient melt flow rate to evenly cover the inner mold surface. At the same time, the molded article must have the necessary physical properties for the end use of the rotational molded article.
Attempts have been made to alter the properties of one polymer by mixing or incorporating a second polymer with the first to form a polymeric blend. For example, secondary polymers have been added to primary polymer compositions to improve impact strength, elongation, tensile properties and even melt flow. Unfortunately, in most of these attempts, the whole blend which is formed is less than the sum of its parts. In other words, often, the property sought by adding the secondary polymer is realized at the expense of one or more other properties or, in fact, the properties sought to be achieved are not attainable. There are many, many patents directed to polymeric blends, but only a handful, at most, of polymeric blends are in the commercial market place.
Among the reasons that polymer blends do not often realize the gain in properties which is sought is due to the incompatibility, both chemically and thermodynamically, between the individual resins which make up the blend and, the inability to provide a uniform mix of the two polymers especially if significant viscosity differences are involved. With regard to compatibility, it has been found that blended polymers which are chemically different, e.g. polar and non-polar, or have different thermodynamic properties such as melting point, viscosity, Tg, etc., tend to segregate in the blend during and after melt blending. Accordingly, the blend is not molecularly uniform and as such articles molded from the polymeric blends do not realize the advantages in physical and/or chemical properties sought or certain properties are sacrificed at the expense of others. It is known to incorporate a compatibilizing agent, specifically synthesized for particular binary polymer blends, to compatibilize the blend. However, such agents are not universal and are not useful outside the scope of the blend for which they were synthesized. Thus, such compatibilizers are expensive and since they represent a third component to a blend, such agents can actually degrade optical, thermal and other physical properties desired for the end use of the molded objects or continuous films.
If it is desired to increase the melt flow rate of a molding resin, it would appear that this could be readily accomplished by simply mixing in a low viscosity material with the higher viscosity molding resin. Unfortunately, differences in the viscosity between polymers renders it extremely difficult to form a uniform mixture. It has been found that mixing polymers of unmatched viscosity also results in a segregation of the polymers, or layering within the mixing device. If it is possible to mix polymers of unmatched viscosity, there is a period of time from the start of mixing until the major, more viscous component of the blend actually becomes the matrix polymer which has uniformly incorporated therein the minor amounts of lower viscosity material. The period of time for the phase inversion, from when the lower viscosity material acts as the matrix to the point that the majority component actually becomes the matrix phase, can greatly reduce productivity.
A study of the effect of viscosity differences on the ability to melt mix polymers was conducted by Chris E. Scott and Sandra K. Joung at the Massachusetts Institute of Technology, Department of Materials Science and Engineering. The results of this study appear in Scott and Joung, Viscosity Ratio Effects in the Compounding of Low Viscosity, Immiscible Fluids into Polymeric Matrices, Polymer Engineering and Science, Vol. 36, No. 12, June 1996 (hereinafter xe2x80x9cScott and Joungxe2x80x9d), the contents of which are incorporated herein by reference.
According to Scott and Joung, many low viscosity, immiscible fluids are difficult to incorporate into polymer matrices because of thermodynamic immiscibility and a large mismatch of melt viscosities. A model system was used in their study to determine the mechanisms and kinetics of mixing in such formulations. The model system consisted of a series of different molecular weight polyethylenes (PE) in polystyrene (PS). The viscosity ratio (major/minor) at 180xc2x0 C. and 100/s was varied from 1.43 to 333. During the study, phase inversion of these formulations in response to compounding was observed. The phase inversion was associated with a transition from low to high mixing torque during compounding. This change was primarily due to an increase in the blend viscosity caused by the morphological transformation. The melting behavior during compounding depended on the melt viscosity of the polyethylene.
According to Scott and Joung, a critical viscosity ratio (minor/major) of 10 exists above which softening of the polystyrene, and thus mixing of the two components, was greatly retarded. Even at very low concentrations, low viscosity polyethylene can have a significant effect on the processing behavior. The phase inversion was represented by a sudden rise in mixing torque. After the phase inversion, the mixing torque remained substantially constant. Notably, even at the high temperature of 200xc2x0 C., it took about five minutes for the phase inversion to occur. At the lower temperatures, it took even longer. The study by Scott and Joung therefore demonstrates that melt mixing of polymers with a viscosity ratio (major/minor) greater than 10 is difficult and time consuming. Such polymers thus are conventionally considered to be practically incompatible.
The study by Scott and Joung also demonstrates that there is no delayed phase inversion when the polymer materials have the same viscosity (i.e. a viscosity ratio of 1) or when the viscosities are sufficiently close to one another. However, when the polymer materials have significant differences in their respective viscosities, a phase inversion is observed in response to prolonged melt mixing. The absence of a delayed phase inversion when a mixture of materials is melt processed, therefore, tends to indicate that the two materials, whether the same or different polymers, are intimately mixed with one another.
A new technology called solid state shear pulverization, developed by the Polymer Technology Center at Northwestern University, converts multi-color, mixed plastics into a homogenous pastel color powder, which is melt processable by all existing plastics fabrication techniques. Molded articles formed from the powder produced by the solid state shear pulverization process have uniform properties. U.S. Pat. No. 5,814,673 issued to Khait describes the solid state shear pulverization process. The entire content of this mentioned patent is herein incorporated by reference. The process of the aforementioned patent involves passing one or more polymeric materials in the solid state through a pulverization device, including modular co-rotating screws fit with only minimal clearance within a barrel. The screw is modified to contain kneading elements, which under shear convert the polymers into a fine powder in the solid state. An important aspect of the process is that cooling is provided to avoid melting so that the polymeric materials always remain in the solid state during the shearing process. The patented solid state shear pulverization process is disclosed as useful for pulverizing two or more incompatible polymers to yield a compatibilized polymeric product, which when molded has uniform properties even in the absence of compatibilizing agents. That the solid state shear pulverization process is capable of providing polymeric particles having good physical properties, uniform color and compatibilization regardless of the differences in the chemical and thermodynamic nature of the polymeric materials which are pulverized, and all done in the solid state, represents a drastic leap forward in processing ordinarily incompatible multicomponent polymeric blends. Heretofore even blending of two polymers required the addition of expensive, specifically synthesized compatibilizing agents, property-enhancing additives or simply could not have been achieved by previous conventional melt-blending techniques. The patented solid state shear pulverization process is particularly useful for processing multi-color, multi-component polymeric scrap for recycle as well as for providing a unique method of blending scrap and/or virgin polymers to provide a polymeric material which is compatibilized without the need for the additional expensive compatibilizing agents.
U.S. Pat. No. 5,814,673 includes numerous examples directed to solid state shear pulverization of multi-component plastic materials including mixtures of two or more of the following polymers; high density polyethylene, linear-and low density polyethylene, polypropylene, polyethylene terephthalate, polystyrene and polyvinyl chloride. Improvements in compatibility between diverse polymers is particularly disclosed in U.S. Pat. No. 5,814,673 as well as forming a uniform colored polymeric powder from multi-component and multi-colored polymer blends. The patent further describes that the pulverized blends have improved tensile strength relative to the physical properties of conventionally melt blended materials. However, the patent is not otherwise specifically concerned with improving the melt flow properties of moldable polymeric resins and at the same time not degrading the other physical properties of the polymeric resin which are necessary for the end use of the molded article which is ultimately formed by melting or which provide sufficient melt strength useful in molding processes. No particular polymeric blends are disclosed for improving melt flow rate, nor is any data for polymeric blends formed by solid state pulverization provided showing actual improvement in melt flow rate without sacrificing melt strength.
In U.S. Ser. No. 09/193,690, the present inventor also discloses that the solid state shear pulverization process as described in U.S. Pat. No. 5,814,673 can be used to mix two or more polymeric materials, including materials of widely diverse viscosity, and provide intimate mixing of the materials including providing a stable microstructure of the formed polymeric material. The inventor discloses that it is sometimes desirable to mix polymeric materials having different viscosities so that a polymer having a high molecular weight and, therefore, high viscosity, will retain high xe2x80x9cmelt strengthxe2x80x9d in film processing, blow molding and the like, and be provided with a lower viscosity as a result of mixing with the lower viscosity polymer by the solid state shear pulverization process.
It is therefore an object of the present invention to provide a moldable polymeric material with an increased melt flow rate without sacrificing the properties of the polymer which render it useful in melt processing techniques.
It is another object of the present invention to provide a moldable polymeric blend without the need for compatibilizing agents so as to improve the melt flow rate and melt strength properties of the blend relative to the properties of the individual polymeric components which form the blend.
It is another object of the present invention to improve the properties of a moldable base polymeric resin by incorporation therein of a secondary polymeric material which will increase the melt flow rate of the base resin without sacrificing the physical and chemical properties of the base resin.
Still another object of the present invention is to provide a method for providing a compatibilized moldable polymeric blend formed from a base resin and a secondary resin and which has improved melt flow properties relative to the base resin.
It is still another object of the present invention to provide a moldable polymeric blend of a base polymer and a secondary polymer whereby the cycle time for molding the polymeric blend is improved relative to the molding cycle time of the base resin.
Still yet another object of the present invention is to provide a process for forming blown film from a polymeric blend which has been mixed and compatibilized by solid state shear pulverization.
A still further object of this invention is to provide a polymeric blend which can be processed into a blown film of reduced thickness relative to the thickness achievable with either of the polymers forming the blend.
These and other objects and modifications of the present invention will become readily apparent from the accompanying description of the invention and appended claims.
To achieve the above object and other objects, the present invention provides a unique process of compatibilizing a first moldable polymer material with a second polymeric material so as to increase the melt flow rate of the first moldable polymer without sacrificing and, often, improving the other chemical and physical properties of the moldable polymer blend. The process comprises the steps of providing at least first and second polymer materials; compatibilizing the polymer material by applying mechanical energy thereto through solid state shear pulverization in the presence of cooling, and discharging the compatibilized particles produced. The cooling is sufficient to maintain the polymer materials in a solid state during the pulverization. The pulverization generates a particulate mixture of the polymer materials, which exhibits a more stable microstructure when annealed than mixtures produced by melt mixing of the polymer materials.
According to another aspect of the present invention, a process of intimately mixing polymer materials is provided so as to improve the melt processing properties of the polymer materials. The process comprises the steps of providing at least first and second polymer materials, applying mechanical energy to the polymer materials through solid state shear pulverization in the presence of cooling, to effect more intimate mixing of the first and second polymer materials than would be provided by melt mixing of those materials, and discharging particles produced by applying the mechanical energy. The cooling is sufficient to maintain the polymer materials in a solid state during the pulverization.
In accordance with this invention, a moldable polymer is provided with an increased melt flow rate by blending therewith a second low viscosity polymer. Blending of the polymers is provided by solid state shear pulverization wherein the polymers are mixed and sheared together without melting. The polymer blend which is formed is intimately mixed into a compatibilized powder which can be molded by any conventional molding process, such as injection, extrusion, rotational and blow molding processes. The polymer blend has an increased melt flow rate without sacrificing the other physical and chemical properties of the moldable polymer. Down-gauging or reduction of film thickness in extrusion or blow-molded films is also achievable with the blends of this invention. The solid state pulverized particulates are melt processable by conventional mold processing to form articles of manufacture, including film, having a substantially homogenous color appearance without color streaking or marbleizing. This color homogeneity is achievable regardless of whether the particulates included mixed color polymeric material of the same or different composition. This avoids the need for the addition of pigments and/or compatibilizing agents to the feedstock and the need to paint the molded or extruded product to hide unpleasant colors and color streaking.
In-situ polymer compatibilization of the polymer blend is evidenced, in one instance, by the resulting pulverized polymeric particulates exhibiting a thermogram different from that of the precursor unpulverized material. For example, the pulverized particulates of the invention exhibit a melting peak and/or crystallization peak quite different from that (those) of the unpulverized material. Moreover, molded articles produced from the pulverized particulates of the invention exhibit increased tensile strengths and lack of delamination upon breaking in mechanical testing, this being a further indication of in-situ polymer compatibilization. Substantially increased elongation is also achieved in many instances which is still a further indication of compatibility.
The present invention is also advantageous in that the pulverized particulates are suitable for direct use as a powder feedstock for powder feedstock-using melt processing techniques without the need for palletizing and pellet grinding operations. Moreover, the polymeric materials can be processed in a manner to achieve in-situ compatibilization of different polymers in a once-through pulverization operation without the need for a compatibilizing agent which are often expensive and specifically synthesized for one particular blend. The pulverized particulates may be mixed with fillers, reinforcing agents, flame retardants, antioxidants and other additives commonly used in the plastics industry if desired.
This invention will now be described with respect to certain embodiments thereof, along with reference to the accompanying illustrations.